Hy Verdigris, please remember to tell us the year of the truck you are making inquiries about. GMCs were positive ground up into the sixties, If your truck has an alternator rather than a generator it may have damaged some diodes, some alternators had an isolation diode at the output, if that is the case with your alternator then no damage would have happened. If you check the colour of the tags on your starter and generator if your truck still has one that should tell you the voltage. red tags are for twelve volt equipment, black tags are for six volt equipment. Your coil may also be marked for voltage, the primary coil circuit should have the positive post of the coil going to the distributor if the truck is positive ground, hope that helps.
